The present invention relates to a fluid pressure control circuit for a working vehicle having a transmission operable by a fluid pressure. To be particular, the control circuit comprises a fluid pressure actuator for controlling the transmission, a fluid supply line for supplying pressure fluid to the actuator, a negative type fluid pressure parking brake connected to the fluid supply line, and control valve means mounted on the fluid supply line. This control valve means has at least a first position to release the pressure fluid from the parking brake and a second position to supply the pressure fluid to the parking brake.
For parking the working vehicle with a known fluid pressure control circuit as noted above, a parking brake control valve acting as the control valve means is operated to a braking position or the first position to release the pressure fluid from the parking brake to a return oil line thereby to actuate the parking brake. At the same time, a change speed control valve is operated to neutral to stop the propelling drive through the actuator and the transmission and to release an excess fluid pressure in the fluid supply line through a relief valve.
According to the known control circuit, therefore, a fluid pressure may be produced in the fluid supply line regardless of the position of the brake control valve. Consequently, the fluid pressure actuator may be moved to a drive position to start the vehicle when the change speed control valve is operated to a forward or backward drive position in a parking state with the brake control valve set to the braking position, namely with a brake release operation being forgotten. This results in a seizure of the negative type parking brake. It is possible to provide a negative brake having a capacity exceeding the propelling force so that the vehicle is braked against movement and the seizure of the brake is avoided even when a vehicle starting operation is effected forgetting a brake release operation. However, such a large capacity negative brake is expensive.